In the field of internal combustion engine, various types of variable compression-ratio mechanisms are previously known. For example, a variable compression-ratio mechanism capable of varying the mechanical compression ratio by varying a relative positional relation between a piston and a cylinder, and a variable compression-ratio mechanism configured to vary a volume of combustion chamber by an auxiliary piston/cylinder are widely known.
On the other hand, Patent Literature 1 discloses that the mechanical compression ratio is reduced by use of such a variable compression-ratio mechanism for a predetermined period from a fuel recovery, in the case that a temperature of exhaust-emission purification catalyst has decreased below a threshold value during a fuel cut. That is, an oxygen storage amount in the catalyst increases during the fuel cut. If the temperature of the exhaust-emission purification catalyst is excessively lowered by this fuel cut, the stored oxygen is not efficiently reduced after the fuel recovery so that a reduction action of NOx is weakened during that interval. In the technique of Patent Literature 1, when the temperature of the exhaust-emission purification catalyst is lowered below the threshold value, the mechanical compression ratio is lowered to promptly increase the temperature of the exhaust-emission purification catalyst. Then, oxygen stored in the catalyst is rapidly consumed to suppress the increase of NOx after the fuel recovery.
In Patent Literature 1, the mechanical compression ratio is reduced to increase the catalyst temperature when the temperature of the exhaust-emission purification catalyst becomes lower than or equal to the threshold value. However, a case that the oxygen storage amount in the exhaust-emission purification catalyst becomes sufficiently large although the temperature decrease of the catalyst is small, such as a case that a duration of the fuel cut is short and a case that a duration of idle stop is short is conceivable. In such a case, the technique of Patent Literature 1 is not effective. Hence, there has been room for improvement regarding the decrease of NOx which is emitted from the internal combustion engine.